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| Scaled reptiles | ||||||||
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| Scientific classification | ||||||||
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 black: range of Squamata | ||||||||
| Suborders | ||||||||
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Squamata (scaled reptiles) is the largest recent order of reptiles, including lizards and snakes. Members of the order are distinguished by their skins, which bear horny scales or shields. They also possess movable quadrate bones, making it possible to move the upper jaw relative to the braincase. This is particularly visible in snakes, which are able to open their mouths very widely to accommodate comparatively large prey. The male members of the group Squamata are the only vertebrates with a hemipenis. This is also the only reptile group in which can be found both viviparous and ovoviviparous species, as well as the usual oviparous reptiles.
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Classically, the order is divided into three suborders:
Of these, the lizards form a paraphyletic group. In newer classifications the name Sauria is used for reptiles and birds in general, and the Squamata are divided differently:
The relationships between these suborders is not yet certain, though recent research suggests that several families may form a hypothetical venom clade which encompasses a majority (nearly 60%) of Squamate species. Fry, B. et al (February 2006). "Early evolution of the venom system in lizards and snakes" (PDF). Nature 439: 584-588. doi:10.1038/nature04328. Named Toxicofera, it combines the following groups from traditional classification:
| Amphisbaenia | |||
| Family | Common Names | Example Species | Example Photo |
|---|---|---|---|
| Amphisbaenidae Gray, 1865 | Tropical worm lizards | Darwin\'s worm lizard (Amphisbaena darwinii) | |
| Bipedidae Taylor, 1951 | Bipes worm lizards | Mexican Mole Lizard (Bipes biporus) | |
| Rhineuridae Vanzolini, 1951 | North American worm lizards | North American worm lizard (Rhineura floridana) | 
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| Trogonophidae Gray, 1865 | Palearctic worm lizards | Checkerboard Worm Lizard (Trogonophis wiegmanni) | |
| Diploglossa | |||
| Family | Common Names | Example Species | Example Photo |
| Anguidae Oppel, 1811 | Glass lizards | Slow Worm (Anguis fragilis) | 
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| Anniellidae Gray, 1852 | American legless lizards | California Legless Lizard (Anniella pulchra) | 
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| Xenosauridae Cope, 1866 | Knob-scaled lizards | Chinese Crocodile Lizard (Shinisaurus crocodilurus) | 
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| Gekkota | |||
| Family | Common Names | Example Species | Example Photo |
| Dibamidae Boulenger, 1884 | Blind lizards | Dibamus nicobaricum | - |
| Gekkonidae Gray, 1825 | Geckos | Thick-tailed Gecko (Underwoodisaurus milii) | 
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| Pygopodidae Boulenger, 1884 | Legless lizards | Burton\'s Snake Lizard (Lialis burtonis) | - |
| Iguania | |||
| Family | Common Names | Example Species | Example Photo |
| Agamidae Spix, 1825 | Agamas | Eastern Bearded Dragon (Pogona barbata) | 
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| Chamaeleonidae Gray, 1825 | Chameleons | Veiled Chameleon (Chamaeleo calyptratus) | 
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| Corytophanidae Frost & Etheridge, 1989 | Casquehead lizards | Plumed Basilisk (Basiliscus plumifrons) | 
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| Crotaphytidae Frost & Etheridge, 1989 | Collared and leopard lizards | Common Collared Lizard (Crotaphytus collaris) | 
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| Hoplocercidae Frost & Etheridge, 1989 | Wood lizards or clubtails | Club-tail Iguana (Hoplocercus spinosus) | - |
| Iguanidae | Iguanas | Marine Iguana (Amblyrhynchus cristatus) | 
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| Leiosauridae Frost et al., 2001 | - | Darwin\'s Iguana (Diplolaemus darwinii) | - |
| Opluridae Frost & Etheridge, 1989 | Madagascan iguanas | Chalarodon (Chalarodon madagascariensis) | - |
| Phrynosomatidae Frost & Etheridge, 1989 | Earless, spiny, tree, side-blotched and horned lizards | Greater Earless Lizard (Cophosaurus texanus) | 
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| Polychrotidae Frost & Etheridge, 1989 | Anoles | Caronlina Anole (Anolis carolinensis) | 
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| Tropiduridae Frost & Etheridge, 1989 | Neotropical ground lizards | (Microlophus peruvianus) | 
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| Platynota | |||
| Family | Common Names | Example Species | Example Photo |
| Helodermatidae | Gila monsters | Gila Monster (Heloderma suspectum) | 
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| Lanthanotidae | Earless Monitor | Earless Monitor (Lanthanotus borneensis) | - |
| Varanidae | Monitor lizards | Perentie (Varanus giganteus) | 
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| Scincomorpha | |||
| Family | Common Names | Example Species | Example Photo |
| Cordylidae | Spinytail lizards | Girdle-tailed Lizard (Cordylus warreni) | 
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| Gerrhosauridae | Plated lizards | Sudan Plated Lizard (Gerrhosaurus major) | 
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| Gymnophthalmidae | Spectacled lizards | - | - |
| Lacertidae Oppel, 1811 | Wall or true lizards | Eyed Lizard (Lacerta lepida) | 
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| Scincidae Oppel, 1811 | Skinks | Western Blue-tongued Skink (Tiliqua occipitalis) | 
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| Teiidae | Tegus or whiptails | Blue Tegu (Tupinambis teguixin) | 
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| Xantusiidae | Night lizards | Granite Night Lizard (Xantusia henshawi) | 
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| Alethinophidia | |||
| Family | Common Names | Example Species | Example Photo |
| Acrochordidae Bonaparte, 1831 | file snakes | Marine File Snake (Acrochordus granulatus) | 
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| Aniliidae Stejneger, 1907Aniliidae (TSN 209611). Integrated Taxonomic Information System. Retrieved on 12 December 2007. | coral pipe snakes | Burrowing False Coral (Anilius scytale) | |
| Anomochilidae Cundall, Wallach and Rossman, 1993.Anomochilidae (TSN 563894). Integrated Taxonomic Information System. Retrieved on 13 December 2007. | dwarf pipe snakes | Leonard\'s Pipe Snake, (Anomochilus leonardi) | |
| Atractaspididae Günther, 1858Atractaspididae (TSN 563895). Integrated Taxonomic Information System. Retrieved on 13 December 2007. | mole vipers | Stiletto Snake (Atractaspis bibroni) | |
| Boidae Gray, 1825Cogger(1991), p.23 | boas | Amazon tree boa (Corallus hortulanus) | 
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| Bolyeriidae Hoffstetter, 1946 | Round Island boas | Round Island Burrowing Boa (Bolyeria multocarinata) | |
| Colubridae Oppel, 1811 | colubrids | Grass Snake (Natrix natrix) | .jpg/58px-Natrix natrix (Marek Szczepanek).jpg)
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| Cylindrophiidae Fitzinger, 1843 | Asian pipe snakes | Red-tailed Pipe Snake (Cylindrophis ruffus) | 
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| Elapidae Boie, 1827 | cobras, coral snakes, mambas, kraits, sea snakes, sea kraits, Australian elapids | King Cobra (Ophiophagus hannah) | 
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| Loxocemidae Cope, 1861 | Mexican burrowing snakes | Mexican burrowing snake (Loxocemus bicolor) | 
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| Pythonidae Fitzinger, 1826 | pythons | Ball python (Python regius) | 
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| Tropidophiidae Brongersma, 1951 | dwarf boas | Northern Eyelash Boa (Trachyboa boulengeri) | |
| Uropeltidae Müller, 1832 | shield-tailed snakes, short-tailed snakes | Cuvier\'s shieldtail (Uropeltis ceylanica) | 
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| Viperidae Oppel, 1811 | vipers, pitvipers, rattlesnakes | European asp (Vipera aspis) | 
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| Xenopeltidae Bonaparte, 1845 | sunbeam snakes | Sunbeam snake (Xenopeltis unicolor) | 
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| Scolecophidia | |||
| Family | Common Names | Example Species | Example Photo |
| Anomalepidae Taylor, 1939 | dawn blind snakes | Dawn Blind Snake (Liotyphlops beui) | |
| Leptotyphlopidae Stejneger, 1892 | slender blind snakes | Texas Blind Snake (Leptotyphlops dulcis) | 
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| Typhlopidae Merrem, 1820Typhlopidae (TSN 174338). Integrated Taxonomic Information System. Retrieved on 13 December 2007. | blind snakes | Black Blind Snake (Typhlops reticulatus) |
Squamates are a monophyletic group that is a sister group to the tuatara, and the group of squamates plus tuatara is a sister group to the group containing other reptiles and birds. Squamate fossils first appear in the early Jurassic, but a mitochondrial phylogeny suggests that they evolved in the late Permian. The evolutionary relationships within the squamates are not yet completely worked out, with the relationship of snakes to other groups being most problematic. From morphological data, Iguanid lizards have been thought to have diverged from other squamates very early, but recent molecular phylogenies, both from mitochondrial and nuclear DNA, do not support this early divergence.Kumazawa, Yoshinori (2007). "Mitochondrial genomes from major lizard families suggest their phylogenetic relationships and ancient radiations". Gene 388: 19-26. doi:10.1016/j.gene.2006.09.026. Because snakes have a faster molecular clock than other squamates, and there aren\'t many early snake and snake ancestor fossils,Lizards & Snakes Alive!. American Museum of Natural History. Retrieved on 2007-12-25. it is difficult to figure out the relationship between snakes and other squamate groups.
Venom is modified saliva, delivered through fangs.Mehrtens (1987), p.243 The fangs of \'advanced\' venomous snakes like viperids and elapids are hollow in order to inject venom more effectively, while the fangs of rear-fanged snakes such as the Boomslang merely have a groove on the posterior edge to channel venom into the wound. Snake venoms are often prey specific, its role in self-defense is secondary. Venom, like all salivary secretions, is a pre-digestant which initiates the breakdown of food into soluble compounds allowing for proper digestion and even "non-venomous" snake bites (like any animal bite) will cause tissue damage.Mehrtens (1987), p.209
Recent research suggests that the evolutionary origin of venom may exist deep in the squamate phylogeny, with 60% of squamates placed in this hypothetical group called Toxicofera. Venom has been known in the families Helodermatidae, Elapidae, Viperidae, and some members of the Colubridae. However, all snakes, some agamid lizards and most monitor lizards are now believed by some to have proteins very closely related to venom.Venom Hunt Finds \'Harmless\' Snakes A Potential Danger. Science Daily. Retrieved on 2007-12-25.Lizards\' poisonous [sic] secret is revealed. NewScientist. Retrieved on 2007-12-25.
See Snakebite
In the US alone, more than 8,000 venomous snake bites are reported each year.First Aid Snake Bites. University of Maryland Medical Center. Retrieved on 2007-12-30. It is estimated that 125,000 people a year die from venomous snake bites.Snake-bites: appraisal of the global situation. Who.com. Retrieved on 2007-12-30. However, large pet constrictors, like boas and pythons, have been known to kill their owners through stangulation on rare occassions. Pet boa constrictor chokes owner. BBC News. Retrieved on 2007-12-30.
Lizard bites, unlike venomous snake bites, are not fatal. The Komodo dragon has been known to kill people due to its size. Komodo dragon kills boy, 8, in Indonesia. msnbc. Retrieved on 2007-12-30. The two known venomous species of lizard, the Gila monster and Mexican beaded lizard have never caused a human death by envenomation.
Even though they survived the worst changes in Earth\'s history, today a lot of squamate species are in endanger due to habitat loss, hunting and poaching, pet trade, alien species, and many others. Because of this, some are in fact extinct with Africa having the most extinct species of squamates. However, breeding programs and wildlife parks are trying to save these reptiles from extinction. Many zoos educate people about the importance of snakes and lizards.
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